EntityTooSmall
AWS EntityTooSmall (Entity Too Small) means a multipart upload part is below the minimum allowed size (except the final part). In Amazon S3, this error returns HTTP 400.
Last reviewed: March 18, 2026|Editorial standard: source-backed technical guidance
What Does Entity Too Small Mean?
S3 rejects multipart upload progress at part validation time, so large-file ingestion stalls until chunk sizing and final-part detection are corrected.
Common Causes
- -A non-final multipart upload part is sent below the minimum part size.
- -Client chunking logic splits payload into overly small parts.
- -Upload final-part detection is incorrect, so a midstream part is treated as non-final and too small.
- -Small objects are sent through multipart upload when single-request PutObject would be appropriate.
How to Fix Entity Too Small
- 1Set multipart part size to at least 5 MB for all non-final parts.
- 2Use PutObject for small files instead of multipart upload.
- 3Re-run upload with a fixed chunker and restart the multipart session if parts are already invalid.
- 4Validate part numbering and final-part detection in the uploader implementation.
Step-by-Step Diagnosis for Entity Too Small
- 1Confirm whether the failing flow uses multipart upload (UploadPart/CompleteMultipartUpload).
- 2Log every part number and byte size; identify any non-final part below minimum size.
- 3Verify the last part is the only part allowed to be under the standard part-size threshold.
- 4Inspect uploader chunking logic for boundary errors during stream flush or resume.
Multipart Part-Size Audit
- -Inspect every UploadPart request and verify non-final parts are >= 5 MiB (example: part 7 at 1.2 MiB fails while not being final).
- -Correlate part-number sequence with stream boundaries (example: resume logic marks a middle chunk as complete-final and emits undersized trailing parts).
Uploader Chunker and Resume Logic
- -Audit chunker implementation for flush behavior under low-buffer conditions (example: periodic flush emits sub-5 MiB chunks during network jitter).
- -Verify retry/resume paths preserve original part boundaries (example: resumed transfer restarts with new chunk size and violates multipart minimum).
How to Verify the Fix
- -Repeat the same upload and confirm EntityTooSmall is no longer returned.
- -Validate part-size telemetry shows all non-final parts at or above minimum threshold.
- -Confirm uploaded object integrity and expected object size after completion.
How to Prevent Recurrence
- -Enforce minimum multipart part size in shared uploader libraries.
- -Add tests for edge cases around stream boundaries, retries, and resume behavior.
- -Track multipart part-size distributions and alert on undersized non-final parts.
Pro Tip
- -lock multipart part size at upload-start metadata and disallow runtime chunk-size changes during retries or resumptions.
Decision Support
Compare Guide
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Official References
Provider Context
This guidance is specific to AWS services. Always validate implementation details against official provider documentation before deploying to production.